Laser with gain medium configured to provide an integrated optical pump cavity

ABSTRACT

An optically-pumped laser having a gain medium configured to provide a low loss, three-dimensional integrated optical pump cavity that substantially confines optical pump radiation within the lasing volume, which is particularly useful for efficiently pumping solid state gain media that has low pump dopant concentration. The integrated pump cavity includes a plurality of boundaries contiguous with the gain medium. An optical pump source such as a laser diode array supplies optical pump radiation that is input into the gain medium through one or more pump cavity windows with a propagation direction transverse to a laser axis defined through the gain medium. In some embodiments, an optical surface is situated opposite the window to approximately recollimate the input pump radiation. The optical pump cavity may be designed to concentrate the optical pump radiation and approximately uniformly pump the entire volume of the lasing medium. In one such embodiment, the pump cavity includes opposing converging surfaces that concentrate the optical pump radiation as it projects along the laser axis. Embodiments are disclosed in which a solid state gain medium has coatings that operate to suppress amplified spontaneous emission (“ASE”). Some embodiments also include heat sinks that directly contact the transverse boundaries and control the temperature distribution within the gain medium in a predetermined manner.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to optically-pumped lasers, andmore particularly, to optically-pumped lasers that have a low pumpenergy absorption per unit length such as may result from a low pumpabsorption cross-section and/or a low doping concentration of pumpabsorptive ions.

2. Description of Related Art

An optically-pumped laser includes a gain medium and an optical pumpsource that supplies optical pump radiation to the gain medium, where itis converted into a laser emission. Many early optically-pumped lasersutilized high intensity arc lamps that were formed into any suitableshape, such as a linear shape or a helically wrapped configuration.Although these sources emit high intensity light, they are “lowradiance” (i.e. they emit over a very large solid angle), and thereforeefficiency was greatly improved by using pump cavities to collect andredirect the pump light to illuminate the laser medium. Accordingly, thelaser medium generally was configured into a long, thin cylinder with alarge side surface area, and its laser axis length was oriented to matchthe length of the arc lamp that pumped it. Early pump cavities utilizedhighly reflecting surfaces surrounding both the arc lamp and the lasermedium, which collected and redirected the pump light through the sideof the laser medium in a multi-pass configuration. Many versions ofthese so-called “side-pumped” or “transverse-pumped” optical pump cavitygeometries were developed and used in laser products.

However, side-pumped laser configurations have many problems, such asconflicting requirements for cooling the laser medium through the mediumside surfaces, suppressing parasitic pump cavity oscillations, andcontrolling the spatial distribution of the optical pump power withinthe gain medium, while still maintaining a high pump energy absorptionefficiency. These problems are particularly difficult when theside-pumped laser medium has a low absorption of pump energy pertransverse pass. In order to obtain high pumping efficiency in such lowabsorption media, the pump energy must be retained by the pump cavityand redirected back through the laser medium many times. When the pumpenergy losses resulting from the successive interactions with thecooling interface, parasitic suppression, and pump cavity opticalsystems become excessive, efficient optical pumping of the side-pumpedlaser medium cannot be achieved and high power laser operation maybecome impossible.

In more recent times high radiance pump sources such as lasers and diodelaser arrays have been developed and utilized as pump sources for manylaser media. Because the light from a high radiance source is emittedover a much smaller solid angle than from an extended lamp source, ahigh radiance pump source can be optically configured into a narrow beamby an optical system. In “end-pumped” or “longitudinal-pumped”configurations, the beam is introduced into the laser medium through oneend and then travels along the laser axis down the length of the gainmedium together with the laser emission. In some mode matchedembodiments of end pumped lasers, the transverse optical pump radiationprofile is matched to the desired transverse mode profile of the laser.In embodiments in which most of the pump energy is absorbed duringtransit along the round trip length of the laser medium (which isusually much larger than the two-pass transverse width of the lasermedium), a pump cavity may not be required to attain high pumpingefficiency.

However, significant problems render high power operation difficult toachieve in end-pumped configurations. For example, excessive opticalpower intensities are created due to the fact that end-pumped lasershave a common propagation axis for both the pump and extracted laserbeams, combined with the fact that a typical laser diode pump beam has ahighly non-uniform transverse intensity distribution. Attempts to designan efficient, practical high power end-pumped laser have encounteredproblems such as excessive optical power intensity due to thecombination of the intensities of the pump and extracted laser beams,severe thermally-induced medium distortion, excessive dopingconcentration or medium length constraints on design optimization,increased laser resonator optical losses resulting from complexmulti-wavelength optical coatings, and spatially non-uniform pumpingdistributions. These problems are particularly severe for embodiments inwhich the laser medium exhibits a high pump saturation flux, such as mayresult from a low pump absorption cross-section.

In addition to these problems, which can limit the performance andincrease the complexity of end-pumped laser embodiments, an additionalproblem arises for an important class of laser media. Specifically,“three-level” or “quasi-three level” laser media exhibit substantialperformance benefits when the product of the medium dopant concentrationand the active volume is minimized, i.e., when the dopant concentrationis low by current standards. This concentration dependence arises due tothe requirement that a substantial upper laser level population densitymust be maintained to overcome the equilibrium lower laser levelpopulation density. One such example of a concentration dependent lasermaterial is ytterbium-doped yttrium aluminum garnet (“Yb:YAG”), whichhas been identified to have potential for use in high power lasers. Inorder to take advantage of such concentration-sensitive media,end-pumped configurations utilizing diode lasers as the pump source havebeen proposed. However, such end-pumped configurations have encounteredthe above-discussed problems, which have restricted scaling them to highaverage power levels.

In order to quantify and compare the performance limitations of theprior art side pumping and end pumping concepts applied to concentrationsensitive laser materials and to provide one measure of performanceimprovement over prior art which is afforded by the utilization of thepresent invention, it may be useful to define a concentration figure ofmerit, F_(c), for the laser medium, which is obtained by dividing theminimum optical pump radiation focal spot area, A_(f), by the product ofthe medium laser axis end area, A, the medium dopant concentration,N_(o), the pump absorption cross-section, σ_(p), and the effectiveend-pumping medium length, L. Symbolically:

F _(c) =A _(f)/(AN _(o)σ_(p) L).  Eq. 1

As used herein, the value of F_(c) provides a quantitative measure ofthe highest efficiency and lowest waste energy that can be realized froma given configuration of a concentration-sensitive medium. Generally, ahigher F_(c) indicates that the laser is more efficient, while a lowerF_(c) indicates that the laser is less efficient.

Prior to the conception and reduction to practice of the presentinvention, after consideration of all prior art shortcomings, it isbelieved that end-pumped configurations have provided the most efficientand scaleable configurations of concentration-sensitive media laserssuch as Yb:YAG. In order to maintain a near optimum level of pump energyabsorption in such configurations, the quantity N_(o)σ_(p)L in the aboveequation must be approximately equal to 1. Since the minimum value for Ais equal to A_(f), it is apparent that for end-pumping embodiments thelargest possible value of the concentration figure of merit is equal to1, which requires that the optical pump radiation completely fill themedium volume. Such optimum end-pumped configurations can be impracticalto implement, and generally must be compromised to a lower F_(c) value.Furthermore, side-pumped embodiments that use external re-entrant pumpcavities have a low pump energy absorption efficiency and/or an F_(c)value significantly lower than 1.0.

SUMMARY OF THE INVENTION

In order to overcome the limitations of the prior art, the presentinvention provides an optically-pumped laser having a gain mediumconfigured to provide a low loss, three-dimensional integrated opticalpump cavity for injected optical pump radiation. The optical pump cavityre-directs the optical pump radiation throughout the lasing volume inmultiple passes, substantially retaining the optical pump radiationwithin the lasing volume to create an average pump absorption lengththat can be configured to be much longer than twice the laser axislength, thereby efficiently extracting energy from the optical pumpradiation even when very low pump absorptive ion dopant concentrationlaser media are employed. The integrated pump cavity can be implementedin lasers having a wide range of power levels, from low power lasers tohigh power lasers that generate one kW or more. Laser embodimentsimplementing the present invention have been demonstrated to have a highefficiency in converting the input pump energy into laser emission whilemaintaining uniform pumping distribution. Particularly, some laserembodiments have a concentration Figure of Merit (F_(c)) greater than1.0, and in some embodiments exceeding 2.0.

An optically-pumped laser apparatus described herein comprises a gainmedium that defines an integrated pump cavity having a plurality ofboundaries contiguous with said gain medium that are reflective of thepump radiation, and an optical system that defines a laser axis througha first end and a second end of the gain medium, which may also bereflective to the pump radiation. An optical pump source supplies theoptical pump radiation, and a beam delivery system is arranged to injectthe optical pump radiation through one or more pump cavity windows intothe gain medium with a propagation direction that is substantially notcollinear with the laser axis. In some embodiments, the boundaries ofthe integrated pump cavity include all boundaries of the laser mediumincluding those situated transverse to said laser axis, and theintegrated pump cavity boundaries are configured so that injectedoptical pump radiation reflects between them while projectinglongitudinally along the laser axis. The optical pump radiation issubstantially contained and absorbed by the laser medium within theintegrated pump cavity, thereby energizing said gain medium to generatea laser emission along said laser axis.

Many such integrated medium/pump cavity configurations can beimplemented. In some embodiments, the optical pump cavity is designed toconcentrate the optical pump radiation and uniformly and efficientlypump the entire volume of the lasing medium without significantlycompromising the attainable beam quality of the laser beam generatedwithin the lasing volume. In one such embodiment, the pump cavityincludes converging surfaces that longitudinally concentrate the opticalpump radiation as it projects along the laser axis.

In some embodiments, the gain medium comprises a solid state gain mediumthat defines a pump cavity including coated boundaries, therebyintegrating the pump cavity with the gain medium. The integrated pumpcavity is particularly useful for high power solid state lasers. Bycarefully designing the pump cavity configuration and appropriatelychoosing the dopant concentration of the active pump ion within thesolid state material, high power optical pump radiation can be absorbedapproximately evenly throughout the volume, producing heat moreuniformly throughout the solid state laser gain medium, which can reduceor substantially eliminate higher order thermal distortion effects thatadversely affect other high power optically-pumped lasers. Also, uniformabsorption reduces or substantially eliminates potentially destructivehot areas within the gain medium. One embodiment includes a lightlydoped (e.g. <1%) solid state laser gain medium such as Yb:YAG, and theintegrated pump cavity includes design features to concentrate theintensity of the optical pump radiation. When pumped to opticaltransparency conditions, the lightly-doped, quasi-three level solidstate laser gain medium produces less heat and lower spontaneousemission losses per unit volume than the other, more highly dopedcrystals that are used in conventional laser designs.

Advantageously, a heat sink can be directly coupled to the transversesurfaces of the solid state gain medium, effectively cooling the gainmedium and optical coating formed thereon, even for high poweroperation. Furthermore, detrimental higher order optical distortioneffects are reduced because the product of the transverse temperaturegradient and the laser axis medium length is minimized through the useof the integrated pump cavity. Also, because the optical pump radiationpropagates substantially orthogonal to the laser axis, coatingrequirements for the end surfaces are simplified and the total intensityproximate to the laser end surfaces is significantly reduced, therebyreducing cost, increasing performance, and improving manufacturingyield.

The foregoing, together with other objects, features and advantages ofthis invention, will become more apparent when referring to thefollowing specification, claims and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this invention, reference is nowmade to the following detailed description of the embodiments asillustrated in the accompanying drawing, wherein:

FIG. 1 is a perspective view of one embodiment of an optically-pumpedlaser including a gain medium and an integrated optical pump cavitydefined by the boundaries of the gain medium;

FIG. 1A is a cross-sectional diagram of the optically-pumped laser ofFIG. 1 taken longitudinally (i.e. parallel with the laser axis),illustrating longitudinal components of optical pump radiationpropagating through a midpoint of the pump cavity window;

FIG. 1B is a transverse cross-sectional view of the optically-pumpedlaser of FIG. 1 taken perpendicular to the laser axis, illustratingselected transverse components of the optical pump radiation propagatingthrough a midpoint of the pump cavity window;

FIG. 1C is a cross-sectional diagram of the optically-pumped laser ofFIG. 1 taken longitudinally (i.e. parallel with the laser axis),illustrating selected longitudinal components of the optical pumpradiation propagating through the pump cavity window at a pointproximate to the upper reflective boundary;

FIG. 1D is a transverse cross-sectional view of the optically-pumpedlaser of FIG. 1, illustrating selected transverse components of theoptical pump radiation propagating through the pump cavity window at apoint proximate to the upper reflective boundary;

FIG. 2 is a longitudinal cross-section of a concentrating, integratedpump cavity according to one embodiment, illustrating one ray of opticalpump radiation concentrating as it projects longitudinally along thelaser axis;

FIG. 3 is a perspective view of one embodiment of the invention,including a solid state laser gain medium in a rectilinear slabgeometric configuration;

FIG. 4 is a transverse cross-sectional diagram of a solid state gainmedium that includes one embodiment of coated surfaces to suppressamplified spontaneous emission (“ASE”);

FIG. 4(a) is a magnified view of a portion of the gain medium depictedin FIG. 4 taken along the left edge thereof.

FIG. 4(b) is a magnified view of a poriton of the gain medium depictedin FIG. 4 taken along the top edge thereof.

FIG. 5 is a transverse cross-sectional diagram of a solid state gainmedium that includes another embodiment of coated surfaces to suppressASE;

FIG. 5(a) is a magnified view of a portion of the gain medium depictedin FIG. 5 taken along the left edge thereof.

FIG. 5(b) is a magnified view of a portion of the gain medium depictedin FIG. 5 taken along the top edge thereof.

FIG. 6 is an embodiment of a solid state laser assembly that includes anend block to suppress ASE.

FIG. 7 is a perspective view of a laser that includes a slab of solidstate gain medium having an inner region doped with a firstconcentration of a pump absorptive ion and an outer region surroundingthe inner region that has a different doping concentration;

FIG. 8 is a cross section of the solid state gain medium shown in FIG.7;

FIG. 9 is an exploded view of a solid state laser including a heat sink;

FIG. 10 is an assembled cross-section of the solid state laser and heatsink shown in FIG. 9;

FIG. 11 is a perspective view of an alternative embodiment in which thesolid state gain medium is formed into an approximately taperedcylinder;

FIG. 12 is perspective view of an alternative embodiment in which thesolid state gain medium comprises an approximately rectangular slabincluding angled ends;

FIG. 13 is perspective view of still another alternative embodiment, inwhich the solid state gain medium comprises an approximately rectangularslab having approximately normal ends;

FIG. 14 is a cross-sectional diagram of the optically-pumped laser ofFIG. 13 taken longitudinally, illustrating four rays of optical pumpradiation input at various points of the pump cavity window;

FIG. 15 is a perspective view of an embodiment of a solid state laserthat comprises an external recollimator situated opposite the pumpcavity window;

FIG. 16 is a block diagram of a laser implemented in a modularconfiguration including multiple solid state modules; and

FIG. 17 is an alternative embodiment of a solid state gain medium withboundaries configured to provide an integrated pump cavity, includingmultiple pump cavity windows to input optical pump radiation.

DETAILED DESCRIPTION

This invention is described in the following description with referenceto the Figures, in which like numbers represent the same or similarelements.

As used herein, the angle of incidence of a light ray with a surface isdefined with respect to the normal (i.e. perpendicular) to the surface.Light rays incident upon a reflective surface are reflected at an anglethat is a function of the angle of incidence. Some surfaces are designedwith materials or optical coatings over a range of angles of incidence.However, even if the surface is not designed to be reflective, undercertain circumstances an interface between two materials will bereflective to light rays having large angles of incidence, an effectcaused by total internal reflection (“TIR”). Specifically, TIR causes alight ray propagating through a material having a high index ofrefraction to be reflected from an interface with another medium thathas a lower index of refraction, if the angle of incidence is largerthan a critical angle. The critical angle for TIR is a function of thedifference between the indices of refraction of the two materials.

The term “optical radiation” is used herein to describe any lightdistribution from a light source. Thus, optical radiation may be definedby a collimated beam, it may be defined by a beam such as a diverging ora converging beam, it may be defined by a collection of light raystraveling at different angles and intensities, or it may be defined by avery complex distribution of light rays. For purposes of illustration,the components of optical radiation may be treated as rays, even thoughthe actual distribution may be more complex.

The term “transverse” is used herein to mean “alongside of” (e.g.surfaces transverse to the laser axis), or “from the side” (e.g.transverse rays approach from the side rather than along the laseraxis). Transverse rays may be, but are not necessarily perpendicular tothe laser axis, and transverse surfaces may be, but are not necessarilyparallel to the laser axis.

Some embodiments of the laser assembly described herein integrate a pumpcavity with a solid state gain medium that has a three-level or quasithree-level lasing transition, such as Yb:YAG (ytterbium-doped yttriumaluminum garnet). Other embodiments may use other solid state gainmedia, such as co-doped gain media in which a first ion (a “pump activeion”) absorbs energy from the optical pump, and transfers this energy toa second ion (a “lasant ion”), which lases.

In many cases, the use of a three-level or quasi three-level gain mediumwith a low doping concentration can provide an efficient high powerlaser. Specifically, “three-level” or “quasi three-level” laser mediaexhibit substantial performance benefits when the product of the mediumdopant concentration and the active volume is minimized. Thisconcentration dependence arises due to the requirement that asubstantial upper level population density must be maintained in anoperating laser to overcome the equilibrium lower laser level populationdensity. Of course, efficient laser operation is highly desirable: inaddition to the energy and cost-saving advantages provided by efficientlaser operation, high efficiency can increase laser output power, reduceheat generation, reduce thermal distortion of the laser emission, andextend the useful life of a laser device. Unfortunately, a low dopingconcentration usually also means that the pump absorption per unitlength is low, which makes it difficult for conventional lasers toefficiently absorb the pump radiation and convert it to stored laserenergy.

FIG. 1 is a perspective view of an optically-pumped laser apparatusincluding an approximately rectangular-shaped gain medium 100 withboundaries, which are configured and/or optically coated appropriately,as described elsewhere, that define a three dimensional, integratedoptical pump cavity. The medium boundaries that also define theintegrated optical pump cavity in FIG. 1 include all side boundaries ofthe gain medium, including an upper boundary 101 and a lower boundary102 opposite thereto, a first lateral boundary 103 and a second lateralboundary 104 opposite thereto, and a first end boundary 111 and a secondend boundary 112 opposite thereto, thereby forming an integrated unit,or a “container” for optical pump radiation that prevents substantialleakage of input optical pump radiation from the integrated pump cavity.However, in some embodiments, the second end boundary 112 may notactually be a significant part of the pump cavity; for example, the pumpcavity may be designed so that the pump radiation will not reach thesecond end, and/or substantially all the pump radiation may be absorbedbefore it reaches the second end. Depending upon the specific design forthe integrated pump cavity, the upper and lower boundaries, the firstand second lateral surfaces, and the first and second end surfaces maybe flat and parallel, or flat and non-parallel, or any otherconfiguration that defines a suitable pump radiation distribution withinthe gain medium.

The gain medium comprises any suitable material and geometric shape. Forexample, in the embodiment described subsequently with reference to FIG.3, the gain medium comprises a solid state gain medium doped with asuitable spatial distribution of one or more pump radiation absorptiveions and lasant ions, such as a Yb:YAG crystal, having an approximatelyrectilinear slab geometric configuration. In alternative embodiments,other suitable gain medium geometric shapes can be utilized, such asapproximately circularly cylindrical or elliptically cylindrical shapes.In other alternative embodiments other gain media can be utilized, suchas a liquid gain medium or a gaseous gain medium, and in suchembodiments the appropriate reflective surfaces may be formed on theimmediate boundaries used to contain the liquid or gas. For example, ifa gaseous medium is used, optical coatings can be formed on the interiorsurface of the container that holds the gas.

A laser axis 114 is defined within the gain medium, through the firstand/or second ends 111 and 112. Optical pump radiation 120, supplied byany suitable optical pump source 122, is injected into the gain mediumin a direction non-parallel with the laser axis 114. The optical pumpsource comprises any suitable high radiance source, such as laserdiodes, laser diode arrays, or other lasers. In operation, the opticalpump radiation 120 provides high intensity optical radiation at awavelength that can be absorbed and converted to upper laser levelenergy storage within the gain medium. The optically-pumped laserapparatus operates by using the stored, converted pump energy togenerate a laser emission within the gain medium.

An optical system 140 is situated on opposite sides of the gain medium,and generally defines an optical extraction system for the laser. Theoptical system 140 defines an optical energy extraction volume for laseremission within the gain medium, and accordingly, the optical systemdefines the laser axis 114 by the central lasing path of the energyextraction volume within the gain medium. So defined, the laser axis 114may be a single straight path through the first and second ends of thegain medium, as illustrated. In alternative embodiments a morecomplicated laser axis could be defined by multiple paths through thegain medium, which may reflect between boundaries, and may opticallyinteract with any other medium boundary or surface. The optical system140 includes suitable components in a configuration suitable for itsdesired purpose, whether it be power extraction, spatial or temporalmode control, and/or waveform control. In some embodiments the first andsecond optical components may be discrete components such as a firstoptical element 141 and a second optical element 142 situated onopposite sides of the gain medium. In other embodiments such as a solidstate laser, the optical system 140 may comprise coatings formed on thefirst and/or second ends of the gain medium, and these coatings,together with the shape of the first and second ends (e.g. flat-flat,curved-flat, or curved-curved) define the extraction volume for thesolid state gain medium. In one alternative, the optical system 140defines an optically resonant cavity, in which case the first and secondoptical elements may comprise first and second end mirrors of a laserresonator or the elements of a ring laser resonator. In anotherembodiment, the optical system may define a beam path to amplify anexternally-generated laser beam (i.e., the gain medium could be used asa one- or multiple-pass amplifier) in which case the first and secondoptical elements may comprise lenses and/or other beam-shaping elements.In some embodiments, the output beam defined by the optical system couldbe a single beam, in alternative embodiments multiple beams could beprovided. In still other embodiments, two or more slabs of the solidstate gain medium 100 can be situated within a single laser resonator,which could be useful to increase output laser power.

A beam delivery system 124 forms the optical pump radiation 120 into oneor more beams shown at 126, which are supplied to an integrated pumpcavity input window 130 formed proximate to the first end of the gainmedium. In alternative embodiments two or more pump cavity windows canbe utilized, and they may be positioned appropriately on any gain mediumboundary. The pump cavity window 130 has a size large enough to acceptthe profile of the pump beam(s) 126 but small enough to reducecontainment losses of injected pump radiation. In one embodiment, thedelivered pump beam profile approximately matches the pump cavitywindow. In many embodiments, the pump cavity window 130 is coated foranti-reflection and anti-absorption at the wavelength of the opticalpump radiation.

The beam delivery system illustrated at block 124 receives the opticalradiation from the pump source and directs it through the pump cavitywindow 130 with an orientation and direction of propagation appropriateto match the selected geometrical configuration of the integrated pumpcavity. Accordingly, the beam delivery system includes any appropriatecomponents such as focusing optics, mirrors, optical fibers, or lensducts to receive the optical radiation, process it as appropriate, anddeliver it through the pump cavity window 130 and into the integratedpump cavity. In one embodiment, the beam delivery system 124 focuses thebeam of optical pump radiation onto the pump cavity window 130 with anangular spread around a normal angle of incidence, and the beam waist(i.e. the narrowest beam width) is approximately coincident with thepump cavity window, thereby providing high intensity optical radiation.In alternative embodiments the angle of incidence may not be normal,and/or the optical pump radiation may not be focused; for example theoptical pump radiation may be provided by a very high radiance sourcethat produces a collimated beam of optical pump radiation, which isinjected at an appropriate angle through the pump cavity window 130 inthe form of a collimated beam without substantial focusing.

In the embodiment of FIG. 1, as a part of the beam delivery system, asmall off-axis cylindrical recollimating reflective surface illustratedgenerally at 135 is provided opposite the pump cavity window 130 tocollimate and deflect the pump beam into the integrated pump cavity witha suitable angular spread. In the embodiment of FIG. 1, therecollimating reflective surface 135 comprises an optical surface 150formed on the lower boundary 102 opposite the pump cavity window. Oneadvantage of this configuration is that the injected pump radiation canbe reflected substantially without loss. However, in another embodimentdisclosed subsequently with reference to FIG. 15, an external off-axiscylindrical reflector can be utilized, positioned adjacent to a smallanti-reflection coated region of the medium boundary opposite the pumpcavity window, which may have manufacturing and cost advantages.

As discussed earlier, the optical pump radiation 120 is injected intothe gain medium through the pump cavity window 130 formed on a boundaryof the gain medium. In FIG. 1, the pump cavity window 130 is formedproximate to the first end of the gain medium, while in otherembodiments the pump cavity window can be formed at another appropriatelocation. In the embodiment of FIG. 1, the optical surface 150 issituated to reflect and process the optical pump radiation after it isinjected through the pump cavity window. As a final optical interfacewith the beam delivery system 124, this optical surface is useful tocontrol the intensity of a pump beam focused through the pump cavitywindow, which would otherwise diverge beyond the plane of the pumpcavity window. The optical surface 150 includes a curved shape having anoptical power to concentrate the optical pump radiation along at leastone axis of its propagation and a linear tilt to provide a longitudinaldeflection of the input pump beam suitable to match the integrated pumpcavity design requirements for the optimum range of injectionpropagation angles. For example, the curved surface may be off-axiscircular or parabolic in a plane parallel with the laser axis, whichconcentrates the optical pump radiation in that plane. In otherembodiments, the curved surface may have a shape to concentrate theinjected optical pump radiation along both axes. In one embodiment theoptical surface has an off-axis cylindrical shape that approximatelycollimates and deflects the optical pump radiation from the pump cavitywindow along the longitudinal axis. As will be described, thisparticular embodiment can be used to maintain a pump radiation intensitythroughout the integrated pump cavity approximately equal to the pumpintensity at the plane of the pump cavity window. In still otherembodiments the optical surface 150 has a lesser optical power that onlyreduces the divergence of the injected optical pump radiation. In someembodiments, such as shown in FIGS. 11-14, the optical surface 150 isco-extensive with the unmodified lower boundary, and has substantiallyzero optical power.

In general, the transverse integrated pump cavity is defined by a lowloss, three-dimensional optical structure that reflects the optical pumpradiation multiple times at a non-zero angle with respect to the laseraxis to produce pump absorption path lengths that can be significantlylonger than twice the medium length along the laser axis, and has aconfiguration to reflect and contain the optical pump radiation withinthe gain medium, while suppressing amplified spontaneous emission (ASE)and parasitic mode oscillation. In broad terms, the integrated opticalpump cavity includes a plurality of reflective surfaces formedcontiguous with (i.e. integrated with) the boundaries of the gainmedium.

After injection within a suitable range of propagation angles into theintegrated pump cavity, the optical pump radiation reflects multipletimes within the pump cavity transverse to the laser axis, whileprojecting longitudinally along the laser axis in a direction away fromthe first end and toward the second end. As a result, the optical pumpradiation pumps the entire length of the gain medium along a total pathlength that can be significantly longer than the longitudinal length ofthe gain medium. In some embodiments, any unabsorbed pump energyreaching the second end is reflected back, so that the pump energy isfully utilized. In many embodiments, the unabsorbed pump energy has alarge enough angle of incidence with the second end that TIR issufficient to reflect it back toward the first end. However, inalternative embodiments, other reflective structures could be utilized.It may be noted that the integrated pump cavity directs optical pumpradiation across (i.e. transverse to) the extraction volume of the laseremission, and does not follow the laser axis. As a result, theelectromagnetic fields of the optical pump radiation and the laseremission do not constructively combine as strongly as for co-linearpropagation, which is particularly useful for high power operation.

Furthermore, the laser emission follows the laser axis, and does notsignificantly interact with the transverse reflective surfaces of thepump cavity. Therefore, as will be described in detail with reference toFIGS. 4, and 5, these transverse reflective surfaces are designed toreflect optical pump radiation and to absorb or otherwise suppress ASEand parasitic mode oscillation. Specifically, as will be described withreference to FIGS. 4, and 5, the integrated pump cavity is designed totake advantage of both angular and spectral reflective differencesbetween the surfaces that transmit or reflect pump radiation and thesurfaces that reflect laser emission in order to suppress ASE andparasitic oscillation. Briefly, in one embodiment, the upper and lowerboundaries are designed to be substantially reflective of the opticalpump radiation over a wide range of angles of incidence about normalincidence, while the first and second lateral boundaries are designed tobe substantially reflective only if the angle of incidence is greaterthan a predetermined angle, which in one embodiment is chosen dependentupon the radiance of the pump source. Generally, a low radiance source(a widely diverging beam) will require a smaller predetermined angle;conversely, a higher radiance source can operate effectively with largerpredetermined angles.

One advantage of distributing the optical pump radiation within theintegrated pump cavity described herein is that the gain medium can havea low active ion concentration-length product (and hence low absorptionper unit path length), which promotes efficient quasi three-level laseroperation. A low active ion concentration is possible because pumpenergy injected into the integrated pump cavity remains substantiallyconfined within the gain medium over absorption path lengthssubstantially longer than twice the longitudinal length of the gainmedium. The containment time of a pump photon within the gain medium,before being absorbed by cavity losses or exiting through the pumpcavity window, is defined as the pump cavity photon lifetime, T_(c). Byproviding a pump cavity structure that increases T_(c), more effectiveutilization can be made of a gain medium having a low active ionconcentration.

Reference is now made to FIGS. 1A, 1B, 1C, and 1D in conjunction withFIG. 1, to describe the operation of the pump cavity to reflect andcontain the optical pump radiation within the gain medium. For highpower embodiments, it is advantageous to provide a high pump intensitydistributed throughout the gain medium to provide the energy necessaryto support high power operation. The pump cavity window transmits thishigh intensity pump radiation into the gain medium, and the integratedpump cavity contains and distributes the high intensity pump radiationthroughout the gain medium.

The intensity distribution of the optical pump radiation within theintegrated pump cavity is determined by a number of factors, includingthe design of the pump cavity, and the cross-sectional shape,divergence, propagation direction, and intensity distribution of theinjected optical pump radiation. Of course, these characteristics willvary between embodiments. In many embodiments, it will be desirable tofocus the optical pump radiation from a laser diode array approximatelyonto the pump cavity window to inject high intensity radiation into thepump cavity. In such embodiments, focused pump radiation from a laserdiode array can be considered as an oval cone of diverging light at eachpoint on the pump cavity window, which begins diverging after it passesthrough the pump cavity window. After the diverging rays pass the windowplane, they then reflect off various surfaces of the integrated pumpcavity, distributing the optical pump radiation through the gain mediumin a complex distribution. Furthermore, in the embodiment of FIG. 1, theoptical surface 150 has a curved shape that concentrates the opticalpump radiation interacting with it, thereby reducing the divergence ofat least some of the injected optical radiation. However in someembodiments, not all of the injected optical pump radiation is reflectedfrom the optical surface 150.

FIGS. 1A-1D show longitudinal and transverse cross-sectional views ofthe pump cavity, which illustrate optical pump radiation reflected fromreflective boundaries including the upper and lower boundaries 101 and102, the first and second lateral boundaries 103 and 104, and the firstand second end boundaries 111 and 112. In some embodiments, the rays atsome boundaries have a angle of incidence large enough to be reflected.due to TIR. In general, each reflection of optical pump radiation has adirectional component related to the angle of incidence of the opticalpump radiation. The optical pump radiation is illustrated herein in theform of selected geometrical components (i.e. “rays”) of optical pumpradiation. For purposes of illustration, the optical pump radiationinjected into the pump cavity is divided into longitudinal componentsdefined in the plane of the laser axis, and transverse componentsdefined in a plane perpendicular to the laser axis. FIGS. 1A and 1C showexemplary longitudinal components, and FIGS. 1B and 1D show exemplarytransverse components. It should be recognized that only a few rays areshown for the purposes of illustration, and that in most embodiments acontinuous distribution of optical radiation will exist incident at thewindow plane and within the integrated pump cavity.

FIGS. 1A and 1B show a bundle of components, or rays, of the opticalpump radiation 120 injected at a midpoint within the pump cavity window130. FIG. 1A shows longitudinal components, FIG. 1B shows transversecomponents. In FIG. 1A, a first ray 161 is injected at normal incidence,a second ray 162 is injected at an angle toward the first end 111, and athird ray 163 is injected at an angle away from the first end 111. Thefirst ray 161 reflects from an angled section of the optical surface 150directly centrally below the pump cavity window, and is injected intothe pump cavity. The second ray 162, having a larger divergence, firstreflects from the first end 111, and then reflects from the opticalsurface 150 into the pump cavity. The third ray 163, also having alarger divergence but in the opposite direction, is reflected from theoptical surface 150.

In FIG. 1B, the transverse components of optical pump radiation injectedat a midpoint in the pump cavity window 130 include a first ray 171injected at a normal incidence, a second ray 172 injected at an angletoward the second lateral surface 104, and a third ray 173 injected atan angle toward the first lateral surface 103. The first ray 171reflects from the lower boundary 102 parallel with the first and secondlateral boundaries, and continues to propagate in a parallel mannerthrough the gain medium. The second ray 172 reflects from the secondlateral boundary 104 (by TIR, for example), then the lower boundary 102,then the first lateral boundary 103, then the upper boundary 101, and soforth until it is absorbed by the medium or reflected from the opposite(second) end 112. The third ray 173 mirrors the path of the second ray,first reflecting from the first lateral boundary 103, then reflectingfrom the lower boundary 102, and so forth.

FIGS. 1C and 1D show a bundle of components, or rays, of the opticalpump radiation 120 injected through the pump cavity window 130 distal(far) from the first end 111. FIG. 1C shows longitudinal components,FIG. 1D shows transverse components. In FIG. 1C, a first ray 181 isinjected at normal incidence, a second ray 182 is injected at an angletoward the first end 111, and a third ray 183 is injected at an angleaway from the first end 111. The first ray 181 reflects from an angledsection of the optical surface 150 directly centrally below the pumpcavity window, and is injected into the integrated pump cavity. Thesecond ray 182, having a larger divergence reflects from the moreinclined area of the optical surface 150 and then is injected into thepump cavity. The third ray 183, also having a larger divergence but inthe opposite direction, is reflected from an area of the optical surface150 near the transition to reflective boundary 102. In practical terms,the angle of the third ray 183 is sufficient to inject it into the pumpcavity.

In FIG. 1D, transverse components of optical pump radiation injectedthrough the pump cavity window 130 proximate to the first lateralboundary 103 include a first ray 191 injected at a normal incidence, asecond ray 192 injected at an angle toward the second lateral surface104, and a third ray 193 injected at an angle toward the first lateralsurface 103. The first ray 191 reflects from the lower boundary 102parallel with the first and second lateral boundaries, and continues topropagate in a parallel manner through the gain medium. The second ray192 reflects from the lower boundary 102, then the second lateralboundary 104, then the upper boundary 101, and so forth until it isabsorbed by the medium or reflected from the opposite (second) end 112.The third ray 193 reflects from the first lateral boundary 103, then thelower boundary 102, then the second lateral boundary 104, and so forth.

The above FIGS. 1A-1D are illustrative only. The relationship betweenthe integrated pump cavity, the optical pump radiation, and the mediumextraction volume is designed to provide an appropriate pump intensitydistribution throughout the laser medium, and many different designs canbe made. In general, each subsequent reflection within the pump cavityhas a reflection angle determined by the angle of incidence with thereflective surface. If the opposing surfaces are parallel, thereflection angle will remain substantially the same upon each subsequentreflection; however if the opposing surfaces are non-parallel, thereflection angle will change, which is described elsewhere in moredetail, for example with reference to FIG. 2, which controls thedistribution of pump energy throughout the integrated pump cavity.

FIG. 2 is a longitudinal cross section of an embodiment of a laserapparatus similar to FIG. 1, and also including a concentratingintegrated pump cavity that includes two opposing non-parallelreflective surfaces defined by the upper medium boundary 101 the lowermedium boundary 102. In FIG. 2, the upper boundary 101 is parallel withthe laser axis 114, and the lower boundary 102 is not parallel, having anon-zero taper angle 205 with respect to the upper boundary 102. Inother embodiments, the upper boundary may have a non-zero taper angle,either in addition to, or instead of the taper in the lower boundary.

The non-parallel reflective surfaces of the concentrating pump cavityhave a suitable optical shape to produce the desired concentrationpattern, for example the surfaces may be flat, approximately flat, orcurved. In the illustrated embodiment, the upper and lower reflectivesurfaces are approximately flat, and the taper angle 205 causes theupper and lower surfaces to operate cooperatively to concentrate theintensity of the optical pump radiation as it travels along the laseraxis 114, and thereby these surfaces approximately compensate for theenergy transfer from the optical pump radiation through absorption bythe gain medium. The taper angle is designed to meet the requirements ofa particular laser design; for example, a large taper angle will morequickly concentrate the optical pump radiation and is more suited forshort cavities and/or highly-doped gain media, while a small taper angleis more suited for longer cavities and/or lightly doped gain media. Insome embodiments, a taper angle that varies along the laser axis eithercontinuously or in steps may be useful. In one embodiment, a constanttaper angle of about 1° has been found to be useful. In someembodiments, the taper angle 205 is large enough that some or all of thepump radiation reverses its direction before reaching the second end112.

To illustrate how the optical pump radiation is concentrated, an opticalpump ray 207 is shown entering the pump cavity at normal incidencethrough the pump cavity window 130. It should be recognized that the ray207 is only one of many rays in the optical pump radiation. After theray 207 is injected into the solid state gain medium, the shape andrelationship of the non-parallel surfaces of the integrated pump cavityat each reflection from the non-parallel lower boundary progressivelyreduces the angle of incidence of the optical pump radiation, increasingspatial overlap between adjacent pump reflections as the optical pumpradiation projects longitudinally along the laser axis from the firstend 111 toward the second end 112, thereby progressively concentratingunabsorbed optical energy. More particularly, as the pump ray 207repeatedly reflects between the upper and lower non-parallel surfaces101 and 102, it has a decreasing longitudinal component along the laseraxis 114. Initially, the angle of reflection from the first and secondreflectors is relatively large and therefore the traversal points of theoptical pump radiation are relatively widely separated as illustrated at221, but because the reflective surfaces are non-parallel, the angle ofreflection becomes progressively narrower as the optical pump radiationprojects longitudinally along the laser axis and therefore the traversalpoints across the laser axis become progressively closer and closer, asillustrated at 222. The increasing spatial closeness of adjacentreflections increasingly concentrates pump energy by increasing theoverlap between adjacent reflections. However, at the same time opticalpump energy is being absorbed during each traversal of the solid stategain medium. The increasingly concentrated intensity at least partiallycompensates for pump energy absorbed or otherwise lost during previoustraversals across the gain medium. In some embodiments, theconcentrating pump cavity can be designed with a shape and taper anglethat approximately compensates for the optical pump energy previouslyabsorbed or lost, so that the absorbed optical pump energy isapproximately uniformly distributed within the gain medium. In summary,the concentrating non-parallel reflective surfaces of the pump cavityshown in FIG. 2 can be designed to more uniformly distribute absorbedpump energy within the extraction volume.

Reference is now made to FIG. 3, which is a perspective view of oneembodiment of laser system in which the gain medium 100 comprises asolid state material having a rectilinear shape configured to provide anintegrated pump cavity. The gain medium defines an integrated pumpcavity that comprises an upper reflectively-coated surface 301, a lowerreflectively-coated surface 302, a first lateral coated surface 303, asecond lateral coated surface 304, a polished and coated first end 311,and a polished and coated second end 312. A laser axis 314 is definedthrough the first and second ends. In FIG. 3, the optical extractionsystem 140 provides a linear laser resonator, and the optical pumpsource 122 includes a laser diode array. The laser system described withreference to FIG. 3 is particularly suited for high power operation(e.g. 1 kilowatt or more); of course this laser system is also useful atlower power levels. One advantage of the solid state embodiment is thata cooling system can be situated proximate to the gain medium where heatis generated without interference with the pump radiation, such asdescribed with reference to FIGS. 9 and 10, which facilitates effectivecooling, an advantage that is especially important at high power. Stillanother advantage is that the gain medium can be mountedstraightforwardly onto a base (not shown) of any laser system.

The optical extraction system 140 in the embodiment of FIG. 3 includes apair of end mirrors including a first end mirror 341 and a second endmirror 342 situated on opposing sides of the gain medium 100. In thisembodiment, the end mirrors define a linear optical laser resonatorthrough the gain medium, with the second end mirror being slightly lessreflective (e.g. 90-95%) to provide the output coupling from the laserresonator for a laser output 343; of course, alternative embodiments mayuse other cavity configurations. The end mirrors may be flat,alternatively they may be curved to enhance stability, or they may haveany other appropriate shape.

The optical system 140 may also include a thermal distortion correctionsystem including a cylindrical lens 345 situated, for example betweenthe first end mirror 341 and the gain medium 300 to approximatelycompensate for thermally-induced optical lensing within the gain medium.Other embodiments may utilize alternative optical configurations asappropriate to compensate for the thermally-induced distortion generatedin the particular system. For example the cylindrical lens, or a higherorder lens may be actively controlled in some embodiments to allowcompensation for varying power levels.

The optical system 140 may also include a mode control system 350situated, for example, between the gain medium and the second endmirror, including a converging lens 351 and a diverging lens 352 spacedapart by a separation 355 to appropriately demagnify the beam within thelaser resonator, and to control the output laser beam 343. Also, themode control system 350 can be utilized to control the lasing modewithin the laser resonator. In some embodiments (not shown), an activesystem could be provided to, for example, adjust the separation 355between the converging and diverging lens, in order to optimize theoutput beam and/or the resonator modes for a particular configurationand power level.

The pump source 122 includes, for example laser diode array 360 thatemits high radiance optical radiation 362 at a wavelength that can beabsorbed and converted to upper laser level storage by the gain medium300. For example, an embodiment of the laser diode array suitable forYb:YAG gain media emits at a wavelength of about 940 nm. The laser diodearray 360 includes any number of laser diodes in any configurationsufficient to provide the necessary optical pumping energy: a largenumber of laser diodes may be needed for high power applications.

A beam delivery system 364 receives the optical radiation from the laserdiode array and optically processes it into a beam 326 having a shapeand orientation suitable for injection into the gain medium through apump cavity window 370. Accordingly, the beam delivery system 364includes any components suitable for its intended purposes such asfocusing optics, cylindrical lenses, optical fibers, or lens ducts. Thepump cavity window 370 is formed proximate to the first end, and issized to accept substantially all of the optical pump radiationdelivered to the pump cavity window by the beam delivery system 364. Inone embodiment, the beam delivery system includes a mirror system toredirect the beam from a remote laser diode pump source, and an asphericfocusing optics or a lens duct to approximately focus the beam 326 ontothe pump cavity window 370. Other embodiments may include an array ofoptical fibers to deliver the beam from the remote laser diode array(s)to the pump cavity window. Advantageously, the beam delivery systeminjects substantially all the optical pump radiation from the laserdiode pump source through the pump cavity window and into the gainmedium, providing a high intensity at the input window, which isprojected throughout the gain medium by the integrated pump cavity.

A curved recollimating surface 375, similar to the optical surface 150(FIG. 1) is formed in the gain medium opposite the pump cavity window370, with a shape that has an off-axis recollimating cylindrical radius,in order to recollimate the input optical pump radiation intoapproximately collimated beams which are then projected over a range ofangles into the integrated pump cavity. So projected, the optical pumpradiation alternately reflects between the first and second reflectivesurfaces, sometimes reflecting from the third and fourth sides at anglesof incidence larger than the critical angle for TIR, while graduallyprojecting along the laser axis down the length of the gain medium.Complete optical pump radiation confinement is further accomplished byreflecting optical pump radiation from the first and second ends 311 and312. Due to the large angle of incidence at the sides 303 and 304 andthe first and second ends 311 and 312, it is expected that the opticalcoatings on these ends will not be required to reflect the optical pumpradiation; however, alternative embodiments may include such reflectivecoatings.

The various surfaces of the solid state gain medium are formed by anysuitable technique such as optical figuring or precision machining. Inone embodiment, the upper and lower reflective surfaces are formed intoflat, opposing, non-parallel (with respect to each other along the laseraxis) surfaces by grinding and polishing techniques. In the dimensionalong the laser axis, the flat surfaces are arranged in a taperedconfiguration, with a larger separation proximate to the first end and aprogressively smaller separation toward the second end. In the dimensionperpendicular to the laser axis, the first and second reflectivesurfaces are equidistant. In FIG. 3, the upper reflective surface 301 isapproximately parallel with the laser axis 314, while the lowerreflective surface 302 is tapered with respect to the laser axis, andtherefore the reflective surfaces are non-parallel with respect to eachother. Specifically, the lower reflective surface 302 is tapered at asmall angle 376 with the central axis 314, such that the included angleis greater than 0° (about 1° in one embodiment). As discussed withreference to FIG. 2, the non-parallelism between the upper and lowersurfaces serves to gradually reduce the angle of incidence of theoptical pump radiation as it projects down the gain medium from thefirst end toward the second end. Accompanying this angular reduction isa corresponding increase in beam overlap which acts to increasinglyconcentrate the optical pump radiation. If desired, the taper angle canbe selected and/or the upper and lower surfaces can be shaped to producean intensity concentration of pump radiation that substantiallycompensates for the intensity reduction due to pump absorption by thelaser medium, thereby producing a substantially uniform average spatialpump intensity distribution throughout a large portion of the gainmedium.

One advantage of the solid state embodiment is that various suitableoptical coatings can be formed directly on the different surfaces of thesolid state gain medium 300. Conventional optical coating techniques areused to provide the necessary coatings on the first and second ends 311and 312. The coatings on the upper and lower surfaces and the first andsecond lateral surfaces are designed to suppress ASE while stillproviding an effective pump cavity, such as described in detail withreference to FIGS. 4 and 5. Generally, the upper and lower reflectivesurfaces 301 and 302 are coated with an optical coating that providesvery high reflectivity at the pump wavelengths for an appropriately widerange of internal angles of incidence about normal. The pump cavitywindow 370 is coated for anti-reflection and anti-absorption at the pumpwavelengths.

In one embodiment the solid state material 100 comprises Yb:YAG(ytterbium-doped YAG) that has a relatively low doping concentrationcompared to other Yb:YAG lasers; for example, doping concentrations inthe range of <0.1% to about 1.0%, are useful for high power laserdesigns, and particularly 0.2% has been used. The concentration isconventionally defined by the percentage of locations in the crystalstructure where Y (yttrium) is replaced by Yb. Of course, alternativeembodiments can utilize other gain media and/or other percentages. Therelatively low doping concentration of Yb:YAG in this embodiment,together with the integrated pump cavity, allows the pump energy to bedistributed throughout the pump cavity, and provides a pump photonlifetime T_(c) substantially longer than the prior art pumping concepts,which is useful to achieve high power operation in three-level and quasithree-level laser media such as Yb:YAG. Furthermore, because much of thewaste heat in a solid state gain medium is created following the samedistribution as the absorbed pump radiation, the relatively low dopingconcentration allows absorption to be distributed throughout the pumpcavity, which promotes a uniform heat distribution throughout the gainmedium. Such thermal considerations are particularly important at highpower, and are discussed in detail with reference to FIGS. 9 and 10.

Suppression of ASE and Parasitic Mode Oscillation

For viable laser operation, the integrated pump cavity described hereinincludes features to suppress gain medium parasitic mode oscillation andamplified spontaneous emission (“ASE”), so that significantamplification of laser emission is allowed only along the optical pathsthat lie along (i.e. parallel with) the laser axis within the extractionvolume and are controlled by the optical system. If any other opticalpath through the gain medium were to permit significant ASE or parasiticmode oscillation, substantial energy would be drawn from the extractionvolume, detrimentally reducing lasing efficiency. For example,significant ASE losses can occur if multiple reflections of the laseremission from the reflective surfaces of the integrated pump cavityprovide a lengthy optical path through the gain medium. Even worse, ifthe optical path of ASE is resonant (i.e. closed), parasitic oscillationcan occur. Unfortunately, if the pump cavity that surrounds the gainmedium were made with standard reflective coatings, laser emission wouldbe reflected around the pump cavity together with pump radiation, givingrise to significant ASE problems. In order to insure effective andefficient laser operation, the integrated pump cavity described hereinincludes special coatings that work individually and cooperatively withthe cavity design to dampen ASE and suppress parasitic mode oscillationwhile still being reflective to the optical pump radiation. Such adesign is particularly useful for laser systems that utilize laser mediahaving high indexes of refraction such as the embodiment that usesYb:YAG as its gain medium.

One novel coating described herein provides control of the effectivecritical angle of total internal reflection (“TIR”), which is selectedto suppress ASE. TIR is a known optical property by which a beampropagating through a material having a high index of refraction isreflected at an interface with another medium that has a lower index ofrefraction, if the angle of incidence is larger than a critical angle.The critical angle at which TIR begins to be observed is a function ofthe difference between the indices of refraction of the two materials,and therefore certain high index materials such as Yb:YAG areparticularly susceptible to TIR-related ASE amplification due to thelarger range of angles that are totally reflected. In certain integratedpump cavity designs without ASE suppression means, reflections due toTIR from the side boundaries could reflect unwanted spontaneous emissionfrom side to side within the pump cavity, amplify the spontaneousemission, drain energy, and provide closed paths to allow parasitic modepower extraction. Conventional multilayer coatings do not increase thecritical angle of TIR.

One ASE suppression system, described with reference to FIGS. 4 and 5,includes specific coatings on all transverse reflective surfaces, whichintroduce sufficient losses at the lasing wavelength so that nosignificant low loss internal optical paths exist that would otherwisesignificantly support ASE or parasitic oscillation at any wavelength forwhich the medium has optical gain.

Reference is now made to FIG. 4 to illustrate one way in which theoptical coatings on the transverse surfaces of the solid state gainmedium 300 are designed to reflect the optical pump radiation and stillsuppress ASE and resultant parasitic oscillation. These transversesurfaces include the upper and lower surfaces 301 and 302, and the firstand second lateral surfaces 303 and 304. Referring briefly to FIGS.1A-1D, it may be noticed that optical pump radiation interacts with thelateral surfaces 103 and 104 only at angles of incidence greater than acritical minimum angle determined by the design of the integrated pumpcavity and the beam characteristics of the optical pump radiationinjected into the pump cavity. For example, in one design, the knownmaximum pump half angle divergence inside the gain medium in thetransverse dimension is about 30°, and therefore the angle of incidenceof the optical pump radiation with the lateral surfaces will generallybe greater than 60°. Beginning with the first lateral surface 303, anoptical coating 400 includes a first layer 401 deposited on the gainmedium 300, the first layer 401 comprising an appropriately thick layerof low loss material that has an index of refraction lower than that ofthe laser medium and small enough to support TIR reflection of opticalradiation at angles greater than the minimum pump angle, but largeenough to accept transmission of optical radiation at lesser angles.Thus, the optical pump radiation, which has an angle of incidencesufficient to be reflected due to TIR, will be reflected at the mediuminterface with the first layer 401, while much of the ASE, which has anangle of incidence less than the minimum pump angle, is mostlytransmitted through the interface and into the first layer 401.

A second appropriately thick layer 402 is deposited on top of the firstlayer 401. The second layer 402 comprises a material having an index ofrefraction at least as high as that of the first layer in order toeliminate TIR at the interface between the first and second layers.Therefore, except for Fresnel reflection, substantially all the ASEtransmitted into the first layer propagates to the second layer 402. Thesecond layer 402 has a high absorption coefficient at the laserwavelength and a sufficient thickness to substantially absorb all ASEthat enters the layer. Opposite the first lateral surface 303, thesecond lateral surface 304 has a similar two-layer coating.

In summary, on both lateral surfaces of the gain medium, the noveltwo-layer coating 400 effectively increases the critical angle for TIRto a value large enough that spontaneous emission generated within thesolid state gain medium will only be reflected from the side boundariesat a large angle along the laser axis. In one embodiment the gain mediumcomprises Yb:YAG, which has an index of refraction of about 1.82. Thecritical angle of incidence for TIR is made greater than about 57° byusing a material for the first coating layer that has an index greaterthan about 1.53. ASE reflected from the lateral boundaries at an anglegreater than 57° and propagating in the longitudinal direction willencounter an angle of incidence with the first or second ends less than33° and will therefore be mostly transmitted out of the gain medium. Oneembodiment of such a coating design for Yb:YAG utilizes aluminum oxide(Al₂O₃) as the first layer material, which effectively sets the criticalangle for TIR greater than about 57°, and in one embodiment,approximately 63°, an increase of about 30° from the typical 33°expected of uncoated or conventionally-coated boundaries. For goodperformance in one embodiment, the first coating layer has a thicknessof at least a few wavelengths thick. Of course, in alternative designsother materials and/or other coating designs could be implemented. Forthe second layer 402, one embodiment comprises germanium (Ge), which ishighly absorptive at the lasing wavelengths near one micron. Inalternative designs other materials and/or other coating designs couldbe implemented.

It will be recognized that other ASE having an angle of incidencegreater than the above-defined effective critical angle for TIR will bereflected from the lateral boundaries, and such ASE may reflect from theupper and lower reflective surfaces 301 and 302. Also, ASE generatedelsewhere within the gain medium may be reflected from the upper andlower surfaces. In order to suppress ASE at the upper and lowersurfaces, spontaneous emission is either absorbed in, or transmittedthrough, the pump cavity surfaces. However, the optical pump radiationmust be highly reflected over a range of angles of incidence aboutnormal incidence, and therefore, unlike the coatings on the lateralsurfaces, the coatings on the upper and lower surfaces do not use TIR toselectively reflect optical pump radiation.

Generally, the coating deposited on the upper and lower surfaces isdesigned to reduce reflection of ASE, while still having highreflectivity of the optical pump radiation. Specifically, awavelength-sensitive reflective coating 410 is deposited on the upperand lower surfaces 301 and 302. The wavelength-sensitive coating 410 isdesigned to substantially suppress ASE on the upper and lower reflectivesurfaces, so that each reflection of ASE incurs a loss significantlygreater than the spontaneous emission and amplification produced withinthe solid angle along the path between successive reflections. In otherwords, the coating 410 is designed so that, over all angles of incidencenot absorbed by the lateral surface coating, the loss at the ASE (laser)wavelength is greater than the operational gain of the gain medium alongthe path between the upper and lower surfaces. For example, if themaximum gain between successive reflections is 10%, then each reflectionof ASE from the upper and lower reflective surfaces incurs at least a10% loss, and preferably as large as possible. Accordingly, the designspecifications for the optical coatings for these pump cavity surfacesshould consider the specific operating conditions of the laser systemand must be selected for each configuration.

Conventional multi-layer coating techniques can be used to design such acoating. Alternatively, one or more layers of selectively (non-pumpabsorbing) ASE-absorbing material can be included in the coating design.In one embodiment, the multilayer coatings on the upper and lowersurfaces are composed of materials substantially all of which have anindex of refraction greater than or approximately equal to that of thefirst layer 401 and an outer layer of a very high loss, high indexmaterial such as germanium, in order to provide very high normalincidence range reflectivity at the pump wavelengths and a reducedreflectivity at the wavelengths for which the laser medium exhibits gaineven at large angles of incidence.

FIG. 5 is a cross-section of an alternative structure for suppressingparasitic oscillation, including an optical coating shown generally at500, including a layer 510 deposited on first lateral surface 303 of thegain medium 300. Like the first layer 401 in the embodiment of FIG. 4,the layer 510 comprises a low loss material having an index ofrefraction lower than that of the gain medium and small enough toreflect optical radiation at angles greater than the minimum pump angle,but large enough to transmit optical radiation at lesser angles. Thus,the optical pump radiation, which has an angle of incidence sufficientto be reflected due to TIR, will be reflected at the interface with thelayer 510, while much of the ASE, which has an angle of incidence lessthan the minimum pump angle, is transmitted through the interface andinto the layer 510. However, the layer 510 will typically be thickerthan the first layer 401. On the outer surface of the layer 510, aruling or some suitable abrasive disfiguration 512 (which may beprovided by sandblasting) is provided to at least partially transmit ASEthat propagates through the layer 510. On the opposite, second lateralsurface 304, a similar optical coating 500 is formed thereon that isreflective to the optical pump radiation due to TIR, and then a rulingor some suitable abrasive disfiguration of the outer surface is made toallow at least partial transmission of spontaneous emission.

FIG. 6 is an alternative embodiment of a solid state laser assemblyhaving an integrated pump cavity, including features for suppressing ASEwithin the pump cavity. In this embodiment, the gain medium 100comprises a solid state gain medium, and the second end 112 does notcomprise an end mirror for reflecting laser emission. The second end 112of the gain medium is bonded by any suitable technique, such asdiffusion bonding, to a rectangular end block 600 that comprises anoptical medium with suitable optical characteristics for transmittingthe laser emission. In one embodiment, the optical medium in the endblock comprises an undoped version of the solid state gain medium; forexample, if the gain medium comprises Yb:YAG, then the optical mediummay comprise undoped YAG. The transverse surfaces of the end block 600include an upper surface 601 and a lower, opposing surface 602, and afirst lateral surface 603 and a second, opposing lateral surfaces 604,which are roughened by any suitable technique such as sandblasting ormade absorptive by suitable optical coatings. The end block 600 includesa first end 610 having a suitable shape for bonding with the second end112. The second end 612, opposite the first end 610, is optically coatedwith a suitable coating. For example, a reflective coating may depositedon the second end 612 to define an end mirror of a laser resonator. Inother embodiments, an anti-reflection coating may be deposited on thesecond end.

ASE suppression in the embodiment of FIG. 6 is provided by the roughenedtransverse surfaces 601, 602, 603, and 604. Particularly, the roughenedsurfaces allow optical radiation to exit the end block 600, andtherefore any ASE incident upon these surfaces will be partiallytransmitted away or absorbed. It may be recognized that any pumpradiation incident upon these roughened surfaces will also betransmitted or lost, and therefore the pump cavity must be designedaccordingly. One way to prevent significant loss of pump radiation is todesign the cavity in a concentrating configuration (such as shown inFIG. 2) with an angle and length sufficient to turn around the pumpradiation before it enters the end block 600. The ASE suppression systemdisclosed with reference to FIG. 6 may be used in combination with otherASE suppression methods, such as those described with reference to FIGS.4 and 5.

Reference is now made to FIGS. 7 and 8 to illustrate an alternativeembodiment of a solid state laser with an integrated pump cavity. FIG. 7shows a solid state laser including a slab of solid state gain medium700 having an inner region 710 and an outer region 720 surrounding theinner region. FIG. 8 is an end view of the solid state gain medium 700shown in FIG. 7. The inner region 710 is doped with an active pumpabsorptive ion at a predetermined level, and the outer region 720 has adifferent doping concentration of the pump ion, which in one embodimentis undoped. Typically, it is advantageous that the outer region has alesser concentration. In many solid state gain media, the pump ion isthe same as the lasant ion, while in other laser media, the pump ion isdifferent from the lasant ion. If the pump ion is different from thelasant ion, the gain media is termed “co-doped”.

The solid state gain medium has an integrated pump cavity that confinesthe optical pump radiation, as described elsewhere herein, for examplewith reference to FIG. 1, 2, 3, or 4, and suppresses ASE and parasiticmodes utilizing appropriate optical coatings formed on the exteriorsurface of the outer region, including an upper reflective surface 732,a lower reflective surface 734, a first lateral surface 736, and asecond lateral surface 738. A first end mirror 740 and a second endmirror 742 define a linear laser resonator within the solid state gainmedium, with a central optical axis 744 defined thereby. The pump source122, described for example with reference to FIG. 1, provides opticalradiation 120 through a pump cavity window 746 on a boundary of thesolid state gain medium.

In embodiments in which the integrated pump cavity produces an averagepump radiation that is approximately uniform and is not significantlydependent upon local variations in the dopant concentrationdistributions, one advantage of the embodiment of FIGS. 7 and 8 is thatthe distribution of stored energy available for laser emission is nearlythe same as the pump absorptive ion dopant distribution. Because thestored energy is concentrated in the inner, more highly doped region andsignificantly reduced in the outer, lesser doped region, the laseremission is concentrated in the inner region, while the laser mode isallowed to extend into the outer, lesser doped region, which reducesboundary dependent losses, thereby increasing efficiency. Particularly,the inner doped region 710 supports a lasing radiation distributionhaving an outline shown in FIG. 8 at 750 extending into the lesser dopedouter region 720, thereby avoiding losses that would otherwise occur ifthe stored energy and lasing radiation distribution extended to theexterior boundary of the laser medium.

Although FIGS. 7 and 8 show an embodiment in which the cross-section ofboth the inner and outer regions is approximately rectangular, otherembodiments may comprise any other suitable cross-section, such ascircular or square. The inner and outer regions of the solid state gainmedium may be formed as a composite structure by bonding suitably dopedmaterials, or alternatively the dopant concentration distributions ofthe inner and outer regions of the solid state gain medium may be formedduring the material fabrication process. In some embodiments, the innerand outer regions may not have a clear boundary; instead the dopingconcentration may be variable, or graded in some manner, between theinner and outer regions. For example, the doping concentration may begraded throughout the gain medium, so that the concentration in thecenter is largest, and decreases progressively away from the center. Inone embodiment both the inner and outer regions of the solid state gainmedium comprise a YAG crystal, the inner region is doped with ytterbium,and the outer region is substantially undoped. It should be apparentthat other embodiments may utilize different solid state gain mediums,different dopants or a combination of dopants. Also other embodimentsmay utilize a graded dopant concentrations, and/or various dopantconcentration distributions.

Cooling a Solid State Gain Medium

Reference is now made to FIGS. 9 and 10 to illustrate a system thatdirectly cools the reflective boundaries of the solid state gain medium.One advantage of the solid state embodiment is that a cooling system canbe situated proximate to the gain medium where heat is generated. Directcooling of the boundaries of the solid state gain medium providessignificant cooling capabilities, which is important for high poweroperation. Prior art side-pumped solid state lasers typically have theirpump reflective surfaces situated separate and apart from the gainmedium. In order to directly cool the boundaries of the solid state gainmedium with these prior art devices, the heat sink is situated betweenthe pump reflective surfaces and the gain medium, which requires theheat sink to be transparent to the pump radiation. Such transparent heatsinks can be difficult to design and expensive to implement. Inaddition, no heat sink is completely transparent, and therefore lossesof optical pump radiation are incurred at each pass through thetransparent heat sink.

In contrast, the integrated pump cavity described herein has reflectivesurfaces formed directly on the solid state gain medium, and thereforesolid, liquid, or gaseous heat sinks can be coupled directly to thesides of the solid state gain medium. Heat can flow directly fromoptical coatings and the gain medium, through the optical coatings intothe heat sinks. Advantageously, the pump light is contained within theboundaries of the gain medium without interacting with the heat sinks,thereby avoiding losses while at the same time providing effective heattransfer.

FIG. 9 is an exploded view of a cooled, high power solid state laserembodiment. In FIG. 9, the solid state gain medium 700 shown in FIG. 7is used as an example of a solid state gain medium; of course, inalternative embodiments other solid state embodiments could be utilized.A first heat sink 900 is coupled to the first lateral surface 736, asecond heat sink 910 is coupled to the second lateral surface 738, athird heat sink 920 is coupled to the upper reflective surface 732, anda fourth heat sink 930 is coupled to the lower reflective surface 734.The third and fourth heat sinks are also thermally coupled to the firstand second heat sinks. The first and second heat sinks 900 and 910preferably comprise a highly heat capacitive material, such as water orthermo-electrically cooled copper, coupled to the respective first andsecond transverse surfaces 736 and 738 using a thermally conductivematerial such as indium foil or a conductive film. The third and fourthheat sinks 920 and 930 comprise any suitable form, such as coppercoupled to respective upper and lower surfaces 732 and 734 and alsocoupled to first and second heat sinks 900 and 910 using a thermallyconductive material such as indium foil or a conductive film. In someembodiments, the first, second, third, and fourth heat sinks comprise anactive or passive heat flow regulation mechanism designed toapproximately provide a desired temperature distribution within the gainmedium. In one embodiment, the heat sinks are designed so that the firstand second heat sinks maintain an approximately constant temperaturealong their interfaces with the gain medium, and the third and fourthheat sinks maintain a predetermined temperature distribution, such asapproximately parabolic. In such embodiment, the thermal distortion ofthe laser emission within the solid state laser medium can be correctedwith a cylindrical lens such as shown in FIG. 3. In some embodiments, itmay be desirable to limit or minimize the temperature variation betweenthe upper and lower medium boundaries, and in such embodiments the thirdand fourth heat sinks may comprise an actively heated or cooled thinmetallic plate.

FIG. 10 is an assembled cross-section of the solid state laser assemblyshown in FIG. 9, in an embodiment where the first and second heat sinks900 and 910 have substantial heat exchange capabilities, such as inembodiments where the heat sinks include active cooling. The third andfourth heat sinks 920 and 930 are situated to draw heat away from theupper and lower surface coatings as illustrated by the arrows 1010 and1020, and conduct it to the first and second active heat sinks 900 and910 where it can be disposed of. Also, the first and second active heatsinks 900 and 910 are situated to draw heat directly away from the firstand second lateral surfaces of the gain medium as illustrated by thearrows 1030 and 1040. In one embodiment, the temperature distributionalong the first and second heat sinks 900 and 910 is designed to matchthe temperature distribution along the adjacent sections of the gainmedium, which advantageously reduces high order thermal distortion.Furthermore, FIG. 10 shows that the first, second, third and fourth heatsinks are all situated immediately adjacent to the boundary of the solidstate gain medium, which provides effective heat transfer.

Additional Alternative Embodiments

The integrated pump cavity described herein can be implemented in manydifferent embodiments. For example, instead of the approximatelyrectangular slab shown in FIG. 3, alternative shapes can be utilized.Some alternative embodiments are shown in FIGS. 11-15, and many othersare possible. Each alternative embodiment illustrated in FIGS. 11-15includes a solid state gain medium having boundaries configured toprovide an integrated pump cavity within the solid state gain medium.Any appropriate optical extraction system is utilized to control thelaser energy within the gain medium, such as the system 140 shown inFIGS. 1 and 3. Furthermore, thermal distortion correction systems andmode control systems may be utilized, such as those discussed withreference to FIG. 3.

Reference is now made to FIG. 11, which is a perspective view of analternative embodiment in which a solid state gain medium, showngenerally at 1100 has an approximately cylindrical shape that defines acylindrical axis 1105, a first end 1111, and a second end 1112, with anapproximately circular cross-section. Alternative embodiments maycomprise other cross-sectional shapes, such as elliptical. In typicalembodiments, laser emission is constrained by an optical extractionsystem such as shown in FIG. 3 to propagate between the first and secondends, approximately following the cylindrical axis 1105. The transversecylindrical boundaries of the gain medium are coated appropriately todefine an integrated pump cavity. For example, coatings may be utilizedas described with reference to FIG. 4 or 5 in order to suppress ASE, bycoating upper and lower surfaces of the cylinder with a pump reflectivecoating, and by coating first and second transverse surfaces with anangle dependent reflective coating. Furthermore, selected surfaces ofthe gain medium may be roughened to suppress ASE as shown in FIG. 6either alone or in combination with the ASE-suppressive coatings of FIG.4 or 5.

Any appropriate source of optical pump radiation 120 may be utilized,such as the laser diode array 360 and beam delivery system 364 shown inFIG. 3. A pump cavity window 1130 is formed proximate to the first end1111 in order to receive the optical pump radiation 120 and input itinto the gain medium. Opposite the pump cavity window 1130, an off-axisrecollimating cylindrical shape 1140 is formed to reflect andapproximately recollimate the optical pump radiation after it is inputthrough the pump cavity window 1130. The approximately recollimated pumpradiation is then reflected between the cylindrical boundaries of theintegrated pump cavity, while gradually reflecting down the length ofthe gain medium. The first and second ends 1111 and 1112 may also beutilized to reflect pump radiation.

In the illustrated embodiment, the cylindrical shape is tapered by anangle 1150, so that the first end has a larger diameter that tapers to asmaller diameter at the second end. Accordingly, the optical pumpradiation within the integrated pump cavity concentrates in apredetermined manner as it traverses the length of the cylinder, such asillustrated and described with respect to FIG. 2.

In order to provide a more efficient laser, the gain medium 1100 in oneembodiment includes an inner region 1160 having an approximatelycylindrical shape doped with an active pump absorptive ion at apredetermined level, and surrounding the inner region, an outer region1170 is formed having a lesser doping concentration. In one embodimentthe outer region has a doping concentration of approximately zero (i.e.the outer region is undoped). Advantageously, the stored energy isconcentrated in the inner, more highly doped region, while the lasermode is allowed to extend into the undoped region, which reduces losses,which allows the lasing mode to be concentrated in the more highly dopedinner region. This configuration can allow better control of the lasingmode within the gain medium.

Reference is now made to FIG. 12, which is a perspective view of a solidstate gain medium shown generally at 1200, with an approximatelyrectangular cross-section but including first and second angled ends1201 and 1202, which are angled with respect to the central axis of therectangular solid. Due to refraction effects, an optical axis 1205defined through this central axis is “bent” at the first and second ends1201 and 1202. The pump cavity in the solid state gain medium 1200 isdefined by an upper reflective surface 1210 and an oppositely-positionedlower reflective surface 1212, and a first and second flat lateralsurface 1220 and 1222 arranged in an opposing parallel relationship. Insome embodiments, both the upper and lower reflective surfaces areapproximately parallel to the laser axis 1205. In other embodiments, theupper reflective surface and/or the lower reflective surface maycomprise a taper angle such as described with reference to FIG. 2 orother suitable shape, which provides a concentrating pump cavity.

In order to inject the optical pump radiation 120 into the gain medium,a pump cavity window 1230 is formed on the upper surface proximate tothe first end 1201. The angled first end 1201 is configured below thepump cavity window so that pump radiation injected within a range ofangles about normal incidence is reflected therefrom and then into thepump cavity. For example, a ray 1235 injected at approximately normalincidence is reflected from the angled first end 1201 and then into thepump cavity.

Reference is now made to FIG. 13, which is a perspective view of anapproximately rectangular slab of solid state gain medium showngenerally at 1300 including a first end 1301 and a second end 1302. Alaser axis 1305 is defined between the first and second ends. Anintegrated pump cavity is defined in the solid state gain medium 1300 bya flat, upper reflective surface 1310, an oppositely-positioned, flat,lower reflective surface 1312, a flat, first lateral surface 1320 and anoppositely-positioned, flat, second lateral surface 1322 that isapproximately parallel to the first lateral surface. In someembodiments, both the upper and lower reflective surfaces areapproximately parallel to the laser axis 1305. In other embodiments, theupper reflective surface and/or the lower reflective surface maycomprise a taper angle or other suitable shape such as described withreference to FIG. 2, which provides a concentrating pump cavity.

A pump cavity window 1330 is formed on the upper reflective surface 1310proximate to a first end 1340, in order to inject an appropriate beam1332 of optical pump radiation 120 supplied from the pump source 122(FIG. 1) through a beam delivery system 1334. It may be noted that theembodiment of FIG. 13, unlike the embodiment of FIG. 1, does not includean optical surface 150 opposite the pump cavity window to recollimatethe pump radiation and inject into the pump cavity. Therefore, if pumpradiation is injected at normal incidence or within a certain range ofinjection angles around normal incidence, the pump beam rays lyingwithin this range of angles will reflect at close to normal incidencefrom the lower surface and then exit through the pump cavity window, andas a result, these rays would be lost and unavailable to pump the gainmedium. In order to prevent such loss of optical pump radiation, thebeam delivery system 1334 is designed to create a beam of optical pumpradiation with a predetermined angular profile, not including nearnormal incidence, so that by reflection from the lower surface and thefirst end, the pump radiation is injected into the integrated pumpcavity.

FIG. 14 is a longitudinal cross-section of the rectangular slab of gainmedium 1300, including rays that illustrate optical radiation injectedinto the integrated pump cavity defined within the gain medium 1300.Four rays, including a first, a second, a third, and a fourth ray 1410,1420, 1430 and 1440 are shown. The first ray 1410 is incident upon thewindow 1330 adjacent to the reflective surface 1310, angled toward thefirst end 1301. The second ray is incident upon the window approximatelyin its center, angled toward the first end at a less steep angle thanthe first ray 1410. Both the first and second rays propagate through thewindow 1330 at an angle toward the first end 1301, then reflect from thefirst end, then reflect from the lower reflective surface 1312, and thenarrive at a point 1450 on the upper reflective surface 1310 adjacent tothe input window 1330. Once at the point 1450, it is clear that thefirst and second rays have been injected into the pump cavity. It can beseen that any pump radiation incident within a first window section 1460between the first and second rays, and having a propagation anglegreater than the first and second rays will be injected into theintegrated pump cavity. However, even if a ray (not shown) is incidenton the first window section, but the angle of incidence is less thanthat of the second ray, that other ray would not be injected into thepump cavity, but would be reflected back out through the window 1330.

The third ray 1430 is incident upon the window 1330 approximately at itscenter, angled toward the pump cavity and the second end 1302. Thefourth ray 1440 is incident upon the window 1330 adjacent to the firstend 1301, and is also angled toward the pump cavity, but at a greaterangle than the second ray 1430. Both the third and fourth rays 1430 and1440 reflect from the lower reflective surface 1312, and then arrive atthe point 1350 adjacent to the window 1330, at which point it is clearthat the third and fourth rays have been injected into the pump cavity.It can be seen that any pump radiation incident within a second windowsection 1470 between the third and fourth rays, and having a propagationangle greater than the third and fourth rays will be injected into theintegrated pump cavity. However, if a ray (not shown) is incident at alocation between the third and fourth rays, but the angle of incidenceis less than that of the third ray, that other ray would not be injectedinto the pump cavity, but would be reflected back out through the window1330.

Using a ray tracing analysis such as shown in FIG. 14, the beam deliverysystem 1334 (FIG. 13) can be designed for any particular embodiment tosupply optical pump radiation to either or both of the first and secondwindow sections 1460 and 1470 with the proper range of propagationangles to ensure that it is injected into the optical cavity. Therefore,the beam delivery system 1334 in FIG. 13 includes any appropriatecomponents such as focusing optics, mirrors, optical fibers, or lensducts to receive the beam from the pump source, process it asappropriate, and deliver it to the appropriate section of the pumpcavity window 1330 with the appropriate directions of propagation.

FIG. 15 is a perspective view of an alternative embodiment of FIG. 13 inwhich the laser assembly also includes an external recollimator 1500situated opposite the pump cavity window 1330 to receive and reflectoptical pump radiation into the integrated pump cavity. An access window1505 is formed on the lower surface opposite the pump cavity window 1330by appropriate coatings (e.g. anti-reflection coatings). The accesswindow 1505 is configured to allow pump radiation injected through thepump cavity window 1330 to propagate through to the externalrecollimator 1500, and to allow optical radiation reflected from theexternal recollimator to be injected back into the gain medium. Forillustrative purposes, the laser assembly in FIG. 15 is shown comprisingthe rectangular slab of FIG. 13; however, the external recollimatingsurface could be utilized with any other embodiment.

The external recollimator 1500 in FIG. 15 comprises a substrate 1510 anda curved reflective surface 1512 formed thereon. In some embodiments thesubstrate 1510 comprises an upper surface having a flat shape suitablefor bonding to a corresponding flat surface of the lower surface 1312 ofthe solid state gain medium, but in other embodiments the substrate maybe spaced apart from the gain medium. For example, a cylindricalreflective lens could be positioned appropriately to provide theexternal recollimator.

In FIGS. 1 and 3 the optical surface 150 was discussed, which in someembodiments can be treated as a recollimator integrated with the lowerboundary. The external recollimator 1500 in FIG. 15 can operate in likemanner as a recollimator integrated with the lower boundary toapproximately recollimates the optical pump radiation injected throughthe pump cavity window. Accordingly, the curved reflective surface 1512has an optical power and shape designed to recollimate or otherwiseoptically process the pump radiation received from the pump cavitywindow to provide the desired propagation direction. Like the integratedrecollimator, the external recollimating surface 1500 allows the opticalpump radiation to be injected at approximately normal incidence, whichis an advantage over the embodiment disclosed in FIGS. 13 and 14. Also,the recollimating surface provides better control of the intensitydistribution within the pump cavity than if it were omitted. Onepotential advantage of the external recollimator over the integratedrecollimator is its cost: the external recollimator may simplifymanufacturing and thereby reduce device cost. However, the integratedrecollimator minimizes losses of optical pump radiation, and thereby theintegrated recollimator provides a more efficient pumping configuration.

FIG. 16 is a block diagram of a modular embodiment of a laser systemthat includes multiple pumped gain medium modules, shown collectively at1600, that operate to amplify a laser beam propagating therethrough. Thegain medium modules include a first module 1601 and a second module 1602arranged in any configuration, such as series, parallel, or acombination of series and parallel as suited for a particularapplication. Additional modules may be situated between the first andsecond modules. Advantageously, as many modules as desired can be addedto generate a high power laser beam, which can be useful for high powerlaser welding, cutting, or drilling.

For purposes of illustration, the elements within only one module willbe described, but it should be apparent that the other modules maycontain similar elements. The first module 1601 incorporates a gainmedium 1610 with the integrated pump cavity disclosed herein, a pumpsource 1612 such as a laser diode array arranged to pump the gainmedium, a power supply 1614, and a control unit 1616 arranged to drivethe power supply and control the laser diode and laser. In someembodiments, each module comprises a separate physical unit, withseparate pump sources, power supplies, and control units. In alternativeembodiments it may be useful to combine one or more of the pump sources,the power supplies, and/or the control unit.

The laser system in FIG. 16 also includes a mode control system 1620that operates to control the mode of the laser beam, such as the modecontrol system 350 shown in FIG. 3, including optical components and anyrelated control systems. The mode control system 1620 may comprisemultiple groups of optical components, each controlling the mode of arespective module; alternatively, the mode control system may comprise asingle group of optical components. The laser system in FIG. 16 alsoincludes a thermal distortion correction system 1630 such as thecylindrical lens 345 shown in FIG. 3 that operates to correct beamdistortions caused by thermal lensing within the gain media in themodules. The thermal distortion correction system may include multiplecomponents, one for each module.

An optical extraction system is situated on either side of the modulesto extract laser energy from the pumped gain media. In one embodiment,the optical extraction system comprises an end mirror 1641 and an outputcoupler 1642 having any suitable configuration to define a laser cavity.An output beam 1643 from the output coupler is directed by any suitableoptical system to its intended use such as welding, drilling, orcutting.

FIG. 17 is an alternative embodiment of a solid state gain medium havingboundaries configured to provide an integrated pump cavity, includingmultiple pump cavity windows to input optical pump radiation into thecavity. For ease of illustration, the embodiment of FIG. 17 resemblesthe embodiment of FIG. 1 in some respects; however it should berecognized that many alternative embodiments are possible. Particularly,the embodiment of FIG. 17 comprises a single slab that resembles acombination of two slabs of the solid state gain medium of FIG. 1connected at their second ends. The embodiment of FIG. 17 utilizesstructures corresponding to those in FIG. 1 including the pump cavitywindow 130, the upper boundary 101, the lower boundary 102, the firsttransverse boundaries 103, the second transverse boundary 104, and therecollimating reflective surface 150. For descriptive purposes, thesolid state gain medium is divided into two regions, a first region 1701and a second region 1702. The first region 1701 includes a pump cavitywindow 130 a, an upper boundary 101 a, a lower boundary 102 a, a firstlateral boundary 103 a, and a second lateral boundary 104 a. The firstregion 1701 is optically pumped by a pump beam 120 a supplied from asuitable pump source 122 a. A recollimating reflective surface 150 a issituated opposite the pump cavity window 130 a. The second region 1702includes similar structures. The multiple pump cavity windows in theembodiment of FIG. 17 allow for pumping by multiple pump beams andmultiple pump sources, which may be useful for injecting more opticalpump power into the pump cavity than possible or practical with a singlepump source input through a single window.

It will be appreciated by those skilled in the art, in view of theseteachings, that alternative embodiments may be implemented withoutdeviating from the spirit or scope of the invention. For example, thepump cavity window may be formed at a non-zero angle with the upperboundary, or it may have a non-flat optical shape. This invention is tobe limited only by the following claims, which include all suchembodiments and modifications when viewed in conjunction with the abovespecification and accompanying drawings.

What is claimed is:
 1. An optically-pumped laser apparatus comprising: again medium having boundaries that define an integrated pump cavityincluding transverse boundaries; an optical system that defines a laseraxis through a first end and a second end of said gain medium; and anoptical pump source that supplies optical pump radiation to said gainmedium in a direction transverse to the laser axis; said integrated pumpcavity including means for concentrating input optical pump radiation asit propagates through the pump cavity and said concentrating meansincluding first and second boundaries of said gain medium that areapproximately non-parallel with respect to each other so that opticalpump radiation reflects between said non-parallel boundaries wherebysaid integrated pump cavity is configured so that said optical pumpradiation is substantially contained within said boundaries and absorbedby said gain medium, thereby energizing said gain medium to generate alaser emission along said laser axis.
 2. The laser apparatus of claim 1,wherein said integrated pump cavity has a configuration for energizingsaid gain medium in an approximately uniform distribution.
 3. The laserapparatus of claim 1 wherein said gain medium comprises anoptically-coated solid state material.
 4. The laser apparatus of claim1, further comprising: a pump cavity window defined at a boundary of thegain medium, said pump window arranged to input said optical pumpradiation into said gain medium; and an optical surface situatedopposite said pump cavity window, said opposing optical surface having aconfiguration to concentrate said input optical pump radiation.
 5. Thelaser apparatus of claim 4 and further comprising means for cooling saidsolid state gain medium to provide an approximately constant temperatureon a first two of said transverse boundaries, and also to provide apredetermined temperature variation on a second two of said transverseboundaries, said cooling means including a heat sink coupled to saidtransverse boundaries of said solid state gain medium.
 6. The laserapparatus of claim 3 wherein said solid state gain medium comprisesYb:YAG having a dopant concentration less than about 1%.
 7. The laserapparatus of claim 3 wherein said solid state gain medium has anapproximately rectilinear configuration.
 8. The laser apparatus of claim3 wherein said solid state gain medium has an approximately cylindricalconfiguration.
 9. The laser apparatus of claim 4, further comprising: asecond optical pump source that supplies second optical pump radiation;a second pump cavity window defined at a boundary of the gain medium,said second pump window arranged to input said second optical pumpradiation into said gain medium; and a second optical surface situatedopposite said second pump cavity window, said second optical surfacehaving a configuration to concentrate said input optical pump radiation.10. The laser apparatus of claim 4 wherein said opposing optical surfaceis formed in said gain medium.
 11. The laser apparatus of claim 4wherein said opposing optical surface is situated outside of said gainmedium.